Over the Internet, a state of each flow is not managed in the network and routing is performed based on a reception address. In a single AS (Autonomous System) managed by each ISP (Internet Service Provider), a packet is transferred on a path in which a total sum of fixed link weights is the smallest according to OSPF (Open Shortest Path First). Since a transfer rate is autonomously determined by a transmission host without flow admission control, link congestion may occur in principle. In order to maintain proper communication quality, one of the key issues for an ISP is to design and manage a NW so that loads of all the links forming the NW are below a proper level. The link load always varies depending on fluctuations in traffic matrix demand and path changes due to various failures. Accordingly, many path control methods are under discussion for distributing link loads by dynamically configuring paths through which packets flow so that a link with a low usage rate is actively used.
Recently, most of the Internet traffic is occupied by HTTP (Hypertext Transfer Protocol) traffic used for Web services. For example, according to the traffic analysis measured on backbone links between Japan and the United States of America from 2006 to 2008, about 60% of traffic is occupied by HTTP packets. In addition, the amount of traffic by UGC (User-Generated Content) such as YouTube (registered trademark) and rich content such as VoD (Video on Demand) grow rapidly. Most of the Internet traffic is occupied by these types of content delivery. Content is typically delivered using a CDN operated by a CDN operator. For example, 74% of the top 1,000 sites in the number of access counts use a CDN. A CDN aims to improve user quality and reduce the amount of traffic in the NW by copying content in a plurality of cache servers distributed over the NW and delivering the content from the cache servers rather than from the original server. Accordingly, the location of an originating node of a delivery flow generated for content delivery is determined according to a content placement scheme and a server selection scheme, and thus user quality and the amount of traffic in the NW are greatly affected by these schemes. A CDN is operated by a CDN operator such as Akamai which is independent of an ISP, and content is delivered from a cache server in an application layer in an overlaid manner. Thus, it is difficult for the ISP to obtain information about the server selection scheme and the content placement scheme. For this reason, path control by the ISP and server selection (or content placement) by the CDN operator are independently performed.
Considerable research has been made for controlling a packet transfer path by an ISP. For example, Forts et al. propose a method of designing link weights for OSPF to minimize a total sum of link costs defined as a function of the amount of traffic which flows through links. Benson et al. propose a method of controlling paths with shorter periodicity based on a short-term prediction of a traffic matrix. Danna et al. formulate, as a linear programming problem, a problem associated with appropriate path control in consideration of a trade-off between fairness of a bandwidth allocated to a user and a usage efficiency of network resources. Kandula et al. propose a method of adaptively determining a path based on a throughput measurement value. In addition, several schemes for controlling a path with a finer degree than a destination address have been proposed. For example, Elwalid et al. focus on MPLS (Multi-Protocol Label Switching) and formulate, as a linear programming problem, a problem of optimally designing LSPs (Label Switched Paths) to minimize the total link cost. Hong et al. and Jain et al. propose a TE (Traffic Engineering) scheme for switching paths in a short period of time with a finer degree. While these schemes assume that an ISP performs path control, a scheme for optimally selecting a path by an end user according to online optimization based on actual measurement information has been also considered. Research for analyzing various path control schemes from the viewpoint of quality of user experience has been also made.
Various research has been also made for a server selection scheme in which a CDN operator selects a server from which content is delivered in response to a delivery request. For example, Poese et al. propose that a CDN operator can appropriately select a server when an ISP provides information about the network state to the CDN operator. In addition to selection of the server, research for optimizing a location of a source server by controlling where content is cached have been also made. For example, Applegate et al. formulate, as a mixed integer programming problem, a problem of determining placement of each content item to minimize the amount of total traffic which flows in the network under constraints of a cache capacity and a link bandwidth. Tang et al. propose a content placement design method for minimizing a replication cost for content placement under constraints of user quality.
For these techniques, refer to Non-Patent Documents 1 and 2.
However, path control by an ISP has an influence on effects of server selection by a CDN operator and server selection by the CDN operator also has an influence on effects of path control by the ISP. In considering the respective control schemes by the ISP and the CDN operator, it is necessary to consider the influences on effects of control by the other party. For example, DiPalantino et al. model both strategies using a game theory and analyze a balance to be realized. Research for cooperatively selecting a server by an ISP and a CDN operator has been also made. For example, Poese et al. mention that an address of a delivery server replied by a DNS (Domain Name System) upon content delivery varies, and a response time will be improved when an ISP selects a server. Frank et al. propose a method of exchanging information necessary to cooperatively select a server between a CDN operator and an ISP.
Jiang et al. analyze, provided that path control by an ISP and server selection by a CDN operator are independently performed, to what extent an optimum state can be approached by exchanging information with each other.